Wednesday, 27 August 2014

After decades of searching, physicists have finally
confirmed the existence of low-energy neutrinos that are direct evidence for
the first crucial step in the nuclear reaction that makes the Sun shine. While
the detection validates well-established stellar fusion theory, future, more
sensitive versions of the experiment could look for deviations from the theory
that would reveal new physics. The
conversion of hydrogen into helium is the source of 99% of the Sun’s energy.
The multistep process begins when the star’s hot, dense core squeezes two
protons together to form deuterium, a heavy isotope of hydrogen with a nucleus
made of one proton and one neutron. One of the fused protons then transforms
into a neutron, a process that releases a neutrino and a positron (the
antimatter counterpart of the electron).

While the positrons are almost instantly annihilated in
collisions with electrons, the neutrinos zip through matter unscathed, so they
escape straight into outer space, radiating in all directions at nearly the
speed of light. Other nuclear reactions in the Sun also produce neutrinos, and
100 billion of the particles bombard each square centimetre of Earth every second. The proton–proton reaction
accounts for 90% of all solar neutrinos, but the neutrinos it emits have
relatively low energy, and their signal can be swamped by the radioactive decay
of ordinary terrestrial materials. Thus, although more-energetic solar
neutrinos have been detected since the 1960s, those from the proton–proton
reaction had eluded detection so far.

Now, the Borexino detector, housed beneath more than a kilometre of rock at the Gran Sasso National Laboratory near L'Aquila,
Italy, has succeeded in detecting the neutrinos that accompany the
proton-proton reaction at the solar core. Physicist Andrea Pocar of the University of
Massachusetts Amherst and his collaborators report the findings in Nature1.

Although solar physicists had a general understanding of
the Sun's nuclear reactions, they could have been mistaken about exactly which
reactions take place and their relative importance. That would have left the
question of how the Sun shines incompletely answered, says Michael Smy, a neutrino
physicist at the University of California, Irvine. For this reason, the Borexino collaboration's
direct detection of the neutrinos “is a landmark achievement”, he says.

Star light, star bright

The finding not only confirms how some 90% of the stars
in the Milky Way — including those similar to the Sun but also many that are
less massive — generate most of their energy, but provides a near-instantaneous
snapshot of the solar core, since the neutrinos arrive at Earth just 8 minutes
after they are created.

The core of the Borexino experiment features a nylon vessel containing 278 tonnes of an ultrapure
benzene-like liquid that emits flashes of light when electrons are scattered by
neutrinos. The liquid was derived from a crude-oil source nearly devoid of
radioactive carbon-14, which can hide the neutrino signal. The detector fluid
is surrounded by 889 tonnes of non-scintillating
liquid that shields the vessel from spurious radiation emitted by the
experiment's 2,212 light detectors.

Borexino can measure the flux of low-energy neutrinos with a
precision of 10%. Future experiments could bring that down to 1%, providing a
demanding test of theoretical predictions and thus potentially uncovering new
physics.

For example, tiny mismatches between the rate of energy
production indicated by neutrino detection and the energy from photons in the
sunlight that reaches Earth could signify the presence of dark matter, the
hypothetical invisible material believed to account for most of the mass in the
Universe, says astrophysicist Aldo Serenelli of the Institute of Space Sciences in Bellaterra, Spain. Experiments
may also be able to test how well models describe the transformation of
electron neutrinos into two other types — tau neutrinos and muon neutrinos — as they
travel from the solar core.

1 comment:

“the experiment could look for deviations from the theory that would reveal new physics”

MY VIEW: it will change basic concept of science

“so they escape straight into outer space, radiating in all directions at nearly the speed of light”

MY VIEW: AGREED but not with the speed of light it travel with variable velocity [may be much higher or much lower] depending on…..

“tiny mismatches between the rate of energy production indicated by neutrino detection and the energy from photons in the sunlight that reaches Earth could signify the presence of dark matter, the hypothetical invisible material believed to account for most of the mass in the Universe”